Ocean displacement map not showing waves
The circle does not have enough geometry.

Displacement in Blender works by modifying existing geometry on the object. Your circle definitely does not have enough existing geometry to make displacement visible. This is an easy fix, just apply a subdivision surface modifier, or select all the edges and subdivide them in the edge context menu.

Additionally, I see that you are in Texture Preview mode. Note that displacement will only show as bump in that mode, and in order to actually displace the mesh you need to switch to rendered view mode in cycles and make sure that displacement and bump are both rendered.